Fresh Particles Research Articles

Micro-Explosion mechanism of iron hybrid Methane-Air premixed flames

Compared to gaseous hydrocarbon flame, a metal hybrid flame has higher energy and even has a higher flame temperature. This study explored the micro-explosion mechanisms of iron particles in methane-air premixed flames. Four different particle sizes were used in this experiment, that is 2–10 μm, 50 μm, 100 μm, and 150 μm. A conical methane-air premixed flame was fixed in a stoichiometric condition and doped with iron particles at various feeding rates. The 50- and 100-μm iron particles had a 20% and 500% higher micro-explosion probability than the 2–10-μm particles. The micro-explosion probability would increase with the increase of particle size. In addition, the micro-explosion probability of the 2–10 μm iron particles was 125% higher under 10% CO addition than under no CO addition. CO plays a critical role in increasing the micro-explosion probability of iron particles smaller than 50 µm.Regarding the mechanism of the micro-explosion of iron particles, the oxidizer diffuses into iron particles and produces iron oxide in a thin shell. However, limited oxygen heterogeneously reacts over the surface of the iron particles and produces Fe3O4. Fe3O4 comprises FeO and Fe2O3. However, when CO reacts Fe3O4, this reaction makes Fe3O4 redox into fresh Fe particles, whereas it will lead to FeO while Fe reacts with O2 predominantly. The presence of FeO brings about iron particles blinking and shrinking right after passing the flame front. When CO was dominated in the reaction, it gave rise to the formation of Fe(CO)5 gas bubbles in the film of iron particles. During the agglomeration of the bubbles, incorporating Fe(CO)5, CO, O2, and N2, the iron particles expand gradually and become hollow spherical particles eventually. However, under special conditions, this particle expansion will deteriorate the micro-explosion of the iron particles.

The chemical composition and mixing state of BC-containing particles and the implications on light absorption enhancement

Abstract. The radiative forcing of black carbon (BC) depends strongly on its mixing state in different chemical environments. Here we analyzed the chemical composition and mixing state of BC-containing particles by using a single-particle aerosol mass spectrometer and investigated their impact on light absorption enhancement (Eabs) at an urban (Beijing) and a rural site (Gucheng) in the North China Plain. While the BC was dominantly mixed with organic carbon (OC), nitrate, and sulfate at both the urban and rural sites, the rural site showed a much higher fraction of BC coated with OC and nitrate (36 % vs. 15 %–20 %). Moreover, the BC mixing state evolved significantly as a function of relative humidity (RH), with largely increased coatings of OC–nitrate and nitrate at high RH levels. By linking with an organic aerosol (OA) composition, we found that the OC coated on BC comprised dominantly secondary OA in Beijing, while primary and secondary OA were similarly important in Gucheng. Furthermore, Eabs was highly dependent on secondary inorganic aerosol coated on BC at both sites, while the coated primary OC also resulted in an Eabs of ∼ 1.2 for relatively fresh BC particles at the rural site. A positive matrix factorization analysis was performed to quantify the impact of different mixing states on Eabs. Our results showed a small Eabs (1.06–1.11) for BC particles from fresh primary emissions, while the Eabs increased significantly above 1.3 when BC was aged rapidly with increased coatings of OC–nitrate or nitrate; it can reach above 1.4 as sulfate was involved in BC aging.

Effect of combustion particle morphology on biological responses in a Co-culture of human lung and macrophage cells

Atmospheric aging of combustion particles alters their chemical composition and morphology. Previous studies have reported differences in toxicological responses after exposure to fresh versus aged particles, with chemical composition being the prime suspect behind the differences. However, less is known about the contribution of morphological differences in atmospherically aged particles to toxicological responses, possibly due to the difficulty in resolving the two properties (composition and morphology) that change simultaneously. This study altered the shape of lab-generated combustion particles, without affecting the chemical composition, from fractal-like to a more compact spherical shape, using a water condensation-evaporation method. The two shapes were exposed to a co-culture of human airway epithelial (A549) and differentiated human monocyte (THP-1) cells at air-liquid interface (ALI) conditions. The particles with different shapes were deposited using an electrostatic field-based ALI chamber. For the same mass dose, both shapes were internalized by cells, induced a pro-inflammatory response (IL-8 and TNFα), and enhanced CYP1A1 gene expression compared to air controls. The more compact spherical particles (representative of atmospherically aged particles) induced more early apoptosis and release of TNFα compared to the more fractal-like particles. These results suggest a contribution of morphology to the increased toxicity of aged combustion-derived particles.

Polarimetric analysis of STEREO observations of sungrazing kreutz comet C/2010 E6 (STEREO)

ABSTRACT Twin STEREO spacecraft pre-perihelion photometric and polarimetric observations of the sungrazing Kreutz comet C/2010 E6 (STEREO) in March 2010 at heliocentric distances 3−28 R⊙ were investigated using a newly created set of analysis routines. The comet fully disintegrated during its perihelion passage. Prior to that, a broadening and an increase of the intensity peak with decreasing heliocentric distance was accompanied by a drop to zero polarization at high phase angles (∼105°–135°, STEREO-B) and the emergence of negative polarization at low phase angles (∼25°–35°, STEREO-A). Outside the near-comet region, the tail exhibited a steep slope of increasing polarization with increasing cometocentric distance, with the slope becoming less prominent as the comet approached the Sun. The steep slope may be attributed to sublimation of refractory organic matrix and the processing of dust grains, or to presence of amorphous carbon. The change in slope with proximity to the Sun is likely caused by the gradual sublimation of all refractory material. The polarization signatures observed at both sets of phase angles closer to the comet photocentre as the comet approached the Sun are best explained by fragmentation of the nucleus, exposing fresh Mg-rich silicate particles, followed by their gradual sublimation. The need for further studies of such comets, both observational and theoretical, is highlighted, as well as the benefit of the analysis routines created for this work.

Enhanced soot particle ice nucleation ability induced by aggregate compaction and densification

Abstract. Soot particles, acting as ice nucleating particles (INPs), can contribute to cirrus cloud formation, which has an important influence on climate. Aviation activities emitting soot particles into the upper troposphere can potentially impact ice nucleation (IN) in cirrus clouds. Pore condensation and freezing (PCF) is an important ice formation pathway for soot particles in the cirrus regime, which requires the soot INP to have specific morphological properties, i.e., mesopore structures. In this study, the morphology and pore size distribution of two kinds of soot samples were modified by a physical agitation method without any chemical modification by which more compacted soot sample aggregates could be produced compared to the unmodified sample. The IN activities of both fresh and compacted soot particles with different sizes, 60, 100, 200 and 400 nm, were systematically tested by the Horizontal Ice Nucleation Chamber (HINC) under mixed-phase and cirrus-cloud-relevant temperatures (T). Our results show that soot particles are unable to form ice crystals at T>235 K (homogeneous nucleation temperature, HNT), but IN is observed for compacted and larger-sized soot aggregates (>200 nm) well below the homogeneous freezing relative humidity (RHhom) for T< HNT, demonstrating PCF as the dominating mechanism for soot IN. We also observed that mechanically compacted soot particles can reach a higher particle activation fraction (AF) value for the same T and RH condition compared to the same aggregate size fresh soot particles. The results also reveal a clear size dependence for the IN activity of soot particles with the same degree of compaction, showing that compacted soot particles with large sizes (200 and 400 nm) are more active INPs and can convey the single importance of soot aggregate morphology for the IN ability. In order to understand the role of soot aggregate morphology for its IN activity, both fresh and compacted soot samples were characterized systematically using particle mass and size measurements, comparisons from TEM (transmission electron microscopy) images, soot porosity characteristics from argon (Ar) and nitrogen (N2) physisorption measurements, as well as soot–water interaction results from DVS (dynamic vapor sorption) measurements. Considering the soot particle physical properties along with its IN activities, the enhanced IN abilities of compacted soot particles are attributed to decreasing mesopore width and increasing mesopore occurrence probability due to the compaction process.

The effect of black carbon aging from NO2 oxidation of SO2 on its morphology, optical and hygroscopic properties

Atmospheric aging of black carbon (BC) leads to changes in its physiochemical properties, exerting complex effects on environment and climate. In this study, we have conducted laboratory chamber experiments to investigate the effects of BC aging on its morphology, hygroscopicity and optical properties by exposing monodisperse fresh BC particles to ambient ubiquitous species of nitrogen dioxide (NO2), sulfur dioxide (SO2) and ammonia (NH3) in absence of UV light. We show a rapid aging from highly fractal to compacted aggregates for the monodisperse BC particles with an initial diameter of 150 nm, with decline in the dynamic shape factor (χ) from about 1.8 to nearly 1. The effective density of the monodisperse BC particles increases from ∼0.54 to 1.50 g cm−3 accordingly. The aging process leads to that the light scattering, absorption, and single scattering albedo of the monodisperse BC particles are strongly enhanced by factors of 7.0, 1.8 and 3.0 respectively. By comparing with the BC aging from other mechanisms, we reveal a critical role of the composition of the coating materials on BC in determining its light absorption enhancement. Moreover, due to strong water uptake capacity of the aged BC particles, the light absorption enhancement (Eabs) could be 40–60% higher at humid atmosphere compared with dry conditions. This BC aging process from NO2 oxidation of SO2 may occur commonly in polluted regions and thus considerably alter its effects on regional air quality and climate.

A clear north-to-south spatial gradience of chloride in marine aerosol in Chinese seas under the influence of East Asian Winter Monsoon

Particulate chloride is a major component of sea salt particles and plays a key role in atmospheric chemistry. Anthropogenic pollutants over the northeastern Asia can be transported to the adjacent seas through the northwest monsoon, which profoundly influences the chloride chemistry over the seas. In this study, spatial distribution of particulate chloride and its sources over the Chinese seas were investigated based on shipboard particle samplings especially online Single Particle Aerosol Mass Spectrometer (SPAMS) over Bohai Sea, North Yellow Sea, and South Yellow Sea (SYS) during a cruise in November 2012. A strong north-to-south (N-S) gradience in marine aerosol composition was found. The Cl−/Na+ ratios in PM2.5 and single particle composition by SPAMS indicated remarkable chloride enrichment in marine aerosol in the north (Bohai Sea), while depletion in southern SYS. The results of size distribution showed that particulate chloride had higher concentration in coarse particles, while the Cl−/Na+ ratio was much higher in submicron particles. In the north (38–40°N), biomass burning, carbonaceous, and Pb-rich type particles had high fractions in all chloride-containing particles identified by SPAMS (on average 66%). Combining chemical composition with back trajectory, it was found that fine-mode chloride enrichment in the north was mainly due to anthropogenic emission especially coal combustion and biomass burning from northern China. However, the high fine-mode chloride depletion in the south (32–34°N) was probably due to acid replacement by sulfate in aged aerosol during atmospheric transport. Our new findings reveal that marine aerosol in Chinese seas would show a clear N-S pattern of more fresh and anthropogenic enriched particles in the north, but more aged aerosol in the south during the East Asia Winter Monsoon, which provides new insights for the quantitative assessment of anthropogenic impact on marine aerosol and future modeling study.

Multiscale investigation of pore network heterogeneity and permeability of fluid catalytic cracking (FCC) particles

Pore network is regarded as one of the most important aspects of FCC (Fluid Catalytic Cracking) catalysts for delivering reactants to active sites and transporting out products, and the structure of which can significantly influence the process efficiency. In this work, six characterization methods complementing each other were employed to study the full-scale pore structure (0.4 nm − 20 µm) of fresh FCC particles, especially the X-ray computed tomography (CT) and focused ion beam-scanning electron microscope (FIB-SEM). To focus on nano-scale pores, 3D reconstruction of a whole FCC particle was achieved based on nano-CT, from which the pore network model (PNM) was successfully extracted. Then, permeability simulations along different directions and through various sub-volumes were carried out to demonstrate the anisotropy and heterogeneity of pore structure, respectively. It was also found that the tortuosity of the pores distributed in the outer layer of the FCC particle was more significant than that in the central part of the particle, which could be the mass transfer limiting region during catalysis. Comprehensive acknowledgment of pore structure provides guidance for the optimization of the design of FCC particles, and the multi-scale characterization strategy is a generic strategy for in-depth investigation of structured porous materials.

Ignition and combustion behavior of single micron-sized iron particle in hot gas flow

This work investigated the ignition and combustion process of single micron-sized iron particles in the hot gas flow of burned methane-oxygen-nitrogen mixture. The particle emission intensity was recorded by a high-speed camera in different flame conditions. Particle temperature was derived through two-color pyrometry method employing an ICCD camera equipped with a stereoscope. Based on scanning electron microscopy (SEM) image, the microstructure change of iron particles was investigated. Results showed that the temperature of burning particle rose rapidly and was much higher than the ambient temperature. The fresh iron particles mainly went through several stages in hot gas flow: heating, melting, rapid combustion and cooling. Some of them became bright again after cooling. According to the above combustion process, combustion parameters including ignition delay time, accelerated burning time, total burning time and second stage of combustion time were defined. All the above defined parameters were almost linearly increasing with the increase of particle size under the same oxygen concentration. For iron particles with roughly the same size, the ignition delay time, accelerated burning time and total burning time decreased as the effective oxygen concentration increased especially for particle size larger than 40 µm. The second stage of combustion time for particles with similar size were almost the same under different oxygen concentrations. After combustion, most of the particles change from irregular shape to spherical or ellipsoidal shape, and some of them showed hollow shell structure. A phenomenon of nano-sized particles releasing during the iron particles combustion had been identified. The frequently observed luminous tail was attributed to coming from the thermal radiation of the formed nanoparticles, which was supported by the SEM sampling analysis of the combustion products.

Overturning CO2 Hydrogenation Selectivity with High Activity via Reaction-Induced Strong Metal-Support Interactions.

Encapsulation of metal nanoparticles by support-derived materials known as the classical strong metal-support interaction (SMSI) often happens upon thermal treatment of supported metal catalysts at high temperatures (≥500 °C) and consequently lowers the catalytic performance due to blockage of metal active sites. Here, we show that this SMSI state can be constructed in a Ru-MoO3 catalyst using CO2 hydrogenation reaction gas and at a low temperature of 250 °C, which favors the selective CO2 hydrogenation to CO. During the reaction, Ru nanoparticles facilitate reduction of MoO3 to generate active MoO3-x overlayers with oxygen vacancies, which migrate onto Ru nanoparticles' surface and form the encapsulated structure, that is, Ru@MoO3-x. The formed SMSI state changes 100% CH4 selectivity on fresh Ru particle surfaces to above 99.0% CO selectivity with excellent activity and long-term catalytic stability. The encapsulating oxide layers can be removed via O2 treatment, switching back completely to the methanation. This work suggests that the encapsulation of metal nanocatalysts can be dynamically generated in real reactions, which helps to gain the target products with high activity.

Uranium Isotopic Composition and Constraints on the Provenance of the Qinghai‐Tibet Plateau's Surface Dust

AbstractThe production and emission of aeolian dust can provide key information on the contemporary and historical links between geological evolution and climate. Here we use 234U/238U isotope ratios and the uranium (U) comminution age method on samples collected from Qinghai‐Tibet Plateau (TP) surface dust and glacier snowpack/cryoconites to determine the erosion and potential transport provenance of the dust, regardless of petrological origin. The spatial variation of the (234U/238U) isotope ratios of TP dust showed relatively high values in the Songpan‐Ganzi‐Hoh Xil, Himalayan and the Kunlun‐Qaidam‐Qilian terranes, while showing relatively low (234U/238U) values in Lhasa and Qiangtang terranes, but among all samples the highest value appeared in the glaciation zone. Dust collected from nearby glacial areas with rugged terrain (e.g., the Qilian and Himalayan Mountains) have higher (234U/238U) values than those collected in the topologically flat non‐glacial areas. These differences are consistent with elevated erosion rates and dust comminution‐transport processes in glacial areas, which yield a major source for large quantities of fresh comminuted particles. Compared to sediments from other regions of the globe, the (234U/238U) values of the typical aeolian sinks (such as loess on the Loess Plateau and ice core dust, etc.) are generally lower in comparison to areas with high elevations where erosion rates are high. The (234U/238U) isotope values of TP dust tend to fall between the high‐elevation sites and the dust sinks, indicating the combined influence of short comminution times (tectonics, landscape dynamics, and glacial erosion) and long residence time on the TP. When compared with other central Asian dust sources, U‐Nd‐Sr isotope signatures in TP dust are distinct and can be thus used as an effective tracer of dust provenance. Using U isotopes, we show that dust originating from the TP undergoes long‐range transport and constitutes potentially significant component of the Asian and Northern Hemisphere atmospheric dust load.

Effects of Volatile Components on Mixing State and Size Distribution of Individual Black Carbon Aerosols

In this study, the effects of different volatile components on the mixing state and size distribution of ambient individual black carbon (BC) aerosols were measured using thermo-denudation with single-particle soot photometry and single-particle aerosol mass spectrometry from September 9 to September 27, 2018, in Shanghai, China. The volatility, chemical composition, size distribution, and core-shell structure of BC aerosol were also analysed. To understand the influence of heating temperature on the BC core-shell structure, the mixing state of ageing particles at room temperature (25°C) and the mixing state of residual material heated to 150°C and 300°C were analysed. BC was internally mixed with secondary components, such as ammonium nitrate, ammonium sulphate, a small amount of sodium nitrate, potassium nitrate, sodium sulphate and potassium sulphate, and other nonvolatile components. At 300°C, the secondary components with low volatility escaped, resulting in a thinner BC coating. However, several low-volatile non-BC substances remained, namely organic species, potassium chloride, potassium nitrate, and other nonvolatile substances. The extremely strong photochemical reaction may have produced large amounts of secondary organic carbon on the BC surface in the afternoon, with the high temperature volatilising the whole particle. However, fresh BC particles emitted from traffic during morning rush hour had ultralow volatility, with relatively fewer low-volatile components on the surface.

The oxidative potential of fresh and aged elemental carbon-containing airborne particles: a review.

Elemental carbon is often found in ambient particulate matter (PM), and it contributes to the PM's oxidative potential (OP) and thus poses great health concerns. Previous review articles mainly focused on the methodologies in evaluating OP in PM and its relationship with selected chemical constituents, including metal ions, PAHs, and inorganic species. In recent years, growing attention has been paid to the effect of atmospheric aging processes on the OP of EC-containing airborne particles (ECCAPs). This review investigates more than 150 studies concerning the OP measurements and physico-chemical properties of both fresh and aged ECCAPs such as laboratory-generated elemental carbon (LGEC), carbon black (CB), soot (black carbon), and engineered carbon-containing nanomaterials (ECCBNs). Specifically, we summarize the characteristics of water-soluble and insoluble organic species, PAHs, quinone, and oxygen-containing functional groups (OFGs), and EC crystallinity. Both water-soluble organic carbon (WSOC) and water-insoluble organic carbon (WIOC) contribute to the OP. Low molecular weight (MW) PAHs show a higher correlation with OP than high MW PAHs. Furthermore, oxidative aging processes introduce OFGs, where quinone (CO) and epoxide (O-C-O) increase the OP of ECCAPs. In contrast, carboxyl (-COOH) and hydroxyl (-OH) slightly change the OP. The low crystallinity of EC favors the oxygen addition and forms active OFG quinone, thus increasing the OP. More detailed analyses for the EC microstructures and the organic coatings are needed to predict the OP of ECCAPs.

Coal and biomass combustion with CO2 capture by CLOU process using a magnetic Fe-Mn-supported CuO oxygen carrier

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a derivative technology of the Chemical Looping Combustion (CLC) process for the combustion of both renewable and non-renewable solid fuels. The present work studies the combustion and CO2 capture efficiency of burning coal or biomass using a single Cu-based oxygen carrier with magnetic properties as an oxygen carrier. The combustion of two different coals and three different biomasses was performed in a 1.5 kWth CLOU continuously operated unit for 36 h. The magnetic properties of the oxygen carrier were also studied after combustion and its separation from the ashes. Complete combustion of the fuel to CO2 and H2O was achieved in all cases. For both coals, CO2 capture efficiency increased with fuel reactor temperature, with both reaching values of 97% at 900 °C. The same increase was obtained with the biomasses. The highest CO2 capture efficiency for a biomass was obtained with pine sawdust, which achieved a value of 93%, followed by almond shell (86%) and olive stone (80%) at 900 °C. Oxygen carrier reactivity remained constant after use in the continuous unit with one small increase in the attrition jet index (AJI) value. The used particles were found to still be as magnetic as the fresh particles, and the fines were also highly magnetic. Values higher than 99% were achieved with separation from the fines reached. It was therefore proven that the elutriated oxygen carrier from the plant was still magnetic and could be easily separated from the elutriated ash.

Wetting properties of fresh urban soot particles: Evaluation based on critical supersaturation and observation of surface trace materials

Soot particles strongly absorb solar radiation and contribute to global warming. Also, wetting properties of soot at emission can affect its lifetime. We investigated surface conditions related to wetting and hydrophobic properties of fresh soot using data from measurements taken in Tokyo. A cloud condensation nuclei (CCN) counter was used to clarify surface conditions of particles composed mainly of water-insoluble (WI) materials: total and active particles as CCN around critical supersaturation (Sc) of 203-nm-diameter WI particles. Averaged number fractions of inactivated particles as CCN at 1.05% supersaturation (SS), which is Sc of hydrophilic WI particles, were estimated as 1.4%. Number fractions of inactive particles changed less at 1.78%SS during rush hour and increased at 0.89%SS, implying that most of the WI particles included small amounts of water-soluble (WS) materials rather than being completely hydrophobic. Based on transmission electron microscope (TEM) analysis of samples collected during rush hour, 69% of the mostly bare soot particles had Na or K small domains that are regarded as originating in fossil fuels. Based on water dialysis analysis results, some Na and K on soot were WS. Combination results with CCN measurements suggest that these WS materials decrease the Sc of soot. Moreover, the morphological structure of sulfate covering Na and K domains on the soot surface implicates pre-existing sodium and potassium compounds on soot as a trigger of soot aging. However, inactive particles at Sc at poor-hydrophilic particles and soot particles composed solely of WI materials on TEM samples were also found, although they were minor. Such particles, which are unfavorable for obtaining a wettable surface, might retain non-hygroscopicity for a longer period in the atmosphere. Evaluation of long-range soot transport can benefit from consideration of slight and inhomogeneous differences of chemical compounds on soot that occur along with their emission.

Evolution under dark conditions of particles from old and modern diesel vehicles in a new environmental chamber characterized with fresh exhaust emissions

Abstract. Atmospheric particles have several impacts on health and the environment, especially in urban areas. Parts of those particles are not fresh and have undergone atmospheric chemical and physical processes. Due to a lack of representativeness of experimental conditions and experimental artifacts such as particle wall losses in chambers, there are uncertainties on the effects of physical processes (condensation, nucleation and coagulation) and their role in particle evolution from modern vehicles. This study develops a new method to correct wall losses, accounting for size dependence and experiment-to-experiment variations. It is applied to the evolution of fresh diesel exhaust particles to characterize the physical processes which they undergo. The correction method is based on the black carbon decay and a size-dependent coefficient to correct particle distributions. Six diesel passenger cars, Euro 3 to Euro 6, were driven on a chassis dynamometer with Artemis Urban cold start and Artemis Motorway cycles. Exhaust was injected in an 8 m3 chamber with Teflon walls. The physical evolution of particles was characterized during 6 to 10 h. Increase in particle mass is observed even without photochemical reactions due to the presence of intermediate-volatility organic compounds and semi-volatile organic compounds. These compounds were quantified at emission and induce a particle mass increase up to 17 % h−1, mainly for the older vehicles (Euro 3 and Euro 4). Condensation is 4 times faster when the available particle surface is multiplied by 6.5. If initial particle number concentration is below [8–9] × 104 cm−3, a nucleation mode seems to be present but not measured by a scanning mobility particle sizer (SMPS). The growth of nucleation-mode particles results in an increase in measured [PN]. Above this threshold, particle number concentration decreases due to coagulation, up to −27 % h−1. Under those conditions, the chamber and experimental setup are well suited to characterizing and quantifying the process of coagulation.

Assessment of real-time bioaerosol particle counters using reference chamber experiments

Abstract. This study presents the first reference calibrations of three commercially available bioaerosol detectors. The Droplet Measurement Technologies WIBS-NEO (new version of the wideband integrated bioaerosol spectrometer), Plair Rapid-E, and Swisens Poleno were compared with a primary standard for particle number concentrations at the Federal Institute for Metrology (METAS). Polystyrene (PSL) spheres were used to assess absolute particle counts for diameters from 0.5 to 10 µm. For the three devices, counting efficiency was found to be strongly dependent on particle size. The results confirm the expected detection range for which the instruments were designed. While the WIBS-NEO achieves its highest efficiency with smaller particles, e.g. 90 % for 0.9 µm diameter, the Plair Rapid-E performs best for larger particles, with an efficiency of 58 % for particles with a diameter of 10 µm. The Swisens Poleno is also designed for larger particles but operates well from 2 µm. However, the exact counting efficiency of the Poleno could not be evaluated as the cut-off diameter range of the integrated concentrator unit was not completely covered. In further experiments, three different types of fluorescent particles were tested to investigate the fluorescent detection capabilities of the Plair Rapid-E and the Swisens Poleno. Both instruments showed good agreement with the reference data. While the challenge to produce known concentrations of larger particles above 10 µm or even fresh pollen particles remains, the approach presented in this paper provides a potential standardised validation method that can be used to assess counting efficiency and fluorescence measurements of automatic bioaerosol monitoring devices.

Gamma-rays from reaccelerated cosmic rays in high-velocity clouds colliding with the Galactic disc

ABSTRACT High-velocity clouds moving towards the disc will reach the Galactic plane and will inevitably collide with the disc. In these collisions, a system of two shocks is produced, one propagating through the disc and the other develops within the cloud. The shocks produced within the clouds in these interactions have velocities of hundreds of kilometres per second. When these shocks are radiative they may be inefficient in accelerating fresh particles; however, they can reaccelerate and compress Galactic cosmic rays from the background. In this work, we investigate the interactions of Galactic cosmic rays within a shocked high-velocity cloud, when the shock is induced by the collision with the disc. This study is focused in the case of radiative shocks. We aim to establish under which conditions these interactions lead to significant non-thermal emission, especially gamma-rays. We model the interaction of cosmic ray protons and electrons reaccelerated and further energized by compression in shocks within the clouds, under very general assumptions. We also consider secondary electron–positron pairs produced by the cosmic ray protons when colliding with the material of the cloud. We conclude that nearby clouds reaccelerating Galactic cosmic rays in local shocks can produce high-energy radiation that might be detectable with existing and future gamma-ray detectors. The emission produced by electrons and secondary pairs is important at radio wavelengths, and in some cases it may be relevant at hard X-rays. Concerning higher energies, the leptonic contribution to the spectral energy distribution is significant at soft gamma-rays.

Age‐Resolved Source and Region Contributions to Fine Particulate Matter During an Extreme Haze Episode in China

AbstractA source‐ and age‐resolved algorithm (SARA) was developed in the CMAQ model and applied to study the sources and ages of primary fine particulate matter (PPM) and secondary inorganic aerosols (SNA) in China during January 2013. Residential and industry are the major contributors to PPM and sulfate, especially for fresh particles. The contributions of power increase with age, accounting for 25% and 51% of aged sulfate and nitrate (>48 hr), respectively. Local emissions and intra‐regional transport with younger age contribute to over 70% of PPM and SNA in Beijing, while long‐range transport from northern China with older age becomes more significant in Shanghai. On pollution days, the ages of PPM and SNA consistently increase, suggesting enhanced contributions from regional transport. This study highlights that the age‐specific particle source and region information produced by our SARA algorithm can help design cost‐effective emission control strategies to reduce extreme haze pollution.

Evolution in physicochemical properties of fine particles emitted from residential coal combustion based on chamber experiment

Fine particles (PM2.5) emitted from residential coal combustion play an important role in atmospheric geochemistry and have adverse effects on human health. Three types of residential coal, i.e., raw-bituminous coal (RBC), raw-anthracite coal (RAC) and honeycomb briquette (HB), were burned using a simulation burning system, and the freshly emitted particles were collected. The mass concentration of PM2.5 emitted from raw coal combustion was higher than HB, and the geochemical composition in PM2.5 varied according to coal types. The fraction of total carbonaceous components in PM2.5 emitted from RBC combustion was the highest (57.0–65.2%), followed by HB (9.6–22.2%) and RAC (11.0–17.6%). SO42− was the dominant water-soluble inorganic ion, accounting for 27.9–35.7% in the total detected water-soluble ions. In order to control sulfate emissions, priority should be given to HB, followed by RBC and RAC. Fe, As and Ba were the major trace elements in emitted PM2.5, accounting for 76.2–83.4% of the total detected trace elements. The freshly emitted individual fresh particles were treated with reactive gases (SO2 and NOX) under high relative humidity (85 ± 5%) in an environmental simulation chamber (ESC). The evolution in the physicochemical properties were tested by electron microscopes. Compared to fresh particles emitted from RBC, RAC and HB combustion, aged particles became more spherical with their average sizes increased by 70.6%, 133.3% and 91.7%, respectively. The relative mass fraction of S element in the particles increased by 13.1%. These results suggest that fresh particles emitted from coal combustion could be aged via aqueous-phase reactions with reactive gases under high humidity. The changes in the particles could negatively impact air quality, and consequently influence the regional climate.